ABS 1990 Classic

Hi I have a small problem with a friends 1990 classic. We have to bleed the ABS. The work shop bible states that you have to bleed the master cylinder. How? I can't see any bleed nipples on the master cylinder apart from the two booster ones on the block. I know all this has to be done exactly by the book so need to know how to do this correctly.
I don't have this problem on my '91. no ABS just jumpy right foot!
Any help appreciated
Steve

Bleeding master cylinder is done by turning on ignition and letting abs pump run. Anything longer than 45seconds and there is still something wrong with system ie air.

Thanks for the info, we are going to do the job tomorrow hopefully all will go well.